The discussion below is merely provided for general background information and is not intended to be used as an aid in determining the scope of the claimed subject matter.
During the manufacture of liquefied gas, for example LNG, often use is made of a liquefaction process using an evaporating refrigerant. During shut down of the liquefaction process (for example when the process plant is subject to repairs or servicing) heat ingress from the environment will lead to evaporation of part of the liquid refrigerant contained inside the refrigerant circuit with concurrent potentially problematic pressure increase. On the other hand, when the liquefaction process is started up after such a period of standstill a fast cooling down of the system and in particular of its main cryogenic heat exchanger (MCHE) sometimes may lead to thermal stresses potentially causing leaks.
The pressure inside both the low pressure part and high pressure part of the liquefaction system depends on the quantity of evaporated refrigerant blocked inside these parts of the liquefaction system. Specifically, during heat up of the system evaporated refrigerant would lead to a pressure increase. By withdrawing part of the evaporated refrigerant such pressure increase is (at least partially) compensated. Withdrawal of evaporated refrigerant to a blow off system is done by opening pressure control valves and at too high pressure by opening safety relief valves.